Ion beam implanters are used to treat silicon wafers with an ion beam. Such treatment can be used to produce n or p type extrinsic materials doping or can be used to form passivation layers during fabrication of an integrated circuit.
When used for doping semiconductors, the ion beam implanter injects a selected ion species to produce the desired extrinsic material. Implanting ions generated from source materials such as antimony, arsenic or phosphorus results in `n type` extrinsic material wafers. If `p type` extrinsic material wafers are desired, ions generated with source materials such as boron, gallium or indium are implanted.
The ion beam implanter includes an ion source for generating positively charged ions from ionizable source materials. The generated ions are formed into a beam and accelerated along a predetermined beam path to an implantation station. The ion beam implanter includes beam forming and shaping structure extending between an ion source and the implantation station. The beam forming and shaping structure maintains the ion beam and bounds an elongated interior cavity or region through which the beam passes while travelling to the implantation station. When operating the implanter, this interior region must be evacuated to reduce the probability of ions being deflected from the predetermined beam path as a result of collisions with air molecules.
Eaton Corporation, assignee of the present invention, currently sells high current implanters under the product designations NV 10, NV-GSD/200, NV-GSD/160, and NV-GSD/80.
Ion sources that generate the ion beams used in the known implanters typically include heated filament cathodes that provide ionizing electrons to the confines of a source chamber. These electrons collide with ion producing materials injected into the source chamber to ionize the materials. These ions exit the source chamber through an exit aperture. After relatively short periods of use, the filament cathodes degrade and must be replaced so that ions can again be generated with sufficient efficiency.
The ionization process for an ion implanter source can also be set up and maintained by transferring power into the source chamber by means of an rf coupling antenna. The antenna is energized by an rf signal that creates an alternating current within the "skin layer" of the conductive antenna. The alternating current in the antenna induces a time varying magnetic field which in turn sets off an electric field in a region occupied by naturally occurring free electrons within the source chamber. These free electrons accelerate due to the induced electric field and collide with ionizable materials within the ion source chamber. The shape of the antenna dictates the shape of the electric field induced within the source chamber. Once the antenna provides a steady state transfer of power into the source chamber, electric currents in the plasma within the ion chamber are generally parallel to and opposite in direction to the electric currents in the antenna. Heretofore, it was not believed the antenna could be immersed directly within the plasma created by delivery of energy from the antenna to the interior of the source chamber. To provide electrical isolation, the antenna was coated with a dielectric material. The dielectric coating tended to erode with use and contaminate the plasma within the source chamber.
Examples of two prior art ion sources are disclosed in U.S. Pat. Nos. 4,486,665 and 4,447,732 to Lenng et al. These two patents disclose ion sources having filaments that provide ionizing electrons within an ion source chamber. These filaments are energized by a direct current power source. Direct currents pass through the filaments and cause electrons to be emitted into the source chamber. These electrons are accelerated to collide with atoms injected into the chamber to create ions for subsequent utilization.